Recently, a low supply voltage has been a major solution for low-power system. However, for a digitally controlled oscillator (DCO) or a voltage-controlled oscillator (VCO), the low supply voltage results in a small swing of an oscillation signal, and a signal-to-noise ratio (SNR) will be worse accordingly. To solve the small swing and worse SNR problem, one solution is using a PMOS-only VCO or an NMOS-only VCO to double the swing, however, the PMOS-only VCO or NMOS-only VCO has high power consumption, and the high swing signal may destroy the transistors manufactured by advanced process.
In addition, another solution is to increase a current to maintain the power of the signal, however, as the current is increased, the inductance must be decreased, and the capacitance needs to be increased accordingly, hence causing some side effect. For example, if the supply voltage changes from 1.6V to 0.8V, the inductance must be 0.25× (i.e. 0.25 times the original inductance), the capacitance needs to increase by 4×. However, the small inductance may seriously degrade its quality factor (Q), and the VCO/DCO needs more power to compensate the performance. In addition, if parallel inductors are designed and the transconductance pair (gm-pair) feeds to a low impedance node to overcome the low quality factor issue, the VCO/DCO also has a parasitic oscillation issue.